1. Field of the Invention
The present invention relates to an image forming apparatus which is capable of forming an image based on information which has been input.
2. Prior Art
Printers such as laser beam printers (LBP), LED printers, LCD printers and so forth which utilize an electrophotographic process generally function to form electrostatic latent images on a photosensitive material corresponding to information to be output, develop the electrostatic latent image and transfer the toner image developed to a recording paper.
Since the characteristics of such printers are that it is possible to perform recording of high quality at high speed and on ordinary papers while generating a low level of noise during recording, they are widely used as output devices for text processing devices and image processing devices. In regard to LBP printers, an electrostatic latent image corresponding to recording information is formed on a photosensitive material by scanning it with a laser beam which is modulated by the recording information, on a photosensitive drum uniformly electrified by a primary electrifier. The recorded image can be transferred onto recording paper by developing the electrostatic latent image using a toner and transferring the toner image to the recording paper.
However, such recording devices involve a disadvantage in that the stroke width of the visible image obtained as the final output is greater than that which may be assumed from the viewpoint of electrical signals when the time taken for radiating a laser beam on information to be output is short, as with a fine line. For example, although a fine line with a width corresponding to one pixel ideally has a width of 106 .mu. (25.4 mm/240) with a resolution of 240 dpi (dot per inch), the stroke width of one pixel on an image is 180 .mu. in some cases. FIG. 1 is a graph representing an example in which deviation from the ideal width value of the stroke width of an image produced by a change in the number of pixels comprising the stroke width is expressed by the percentage obtained by dividing the stroke width of an image by the ideal width value. As can be seen from the graph, when a stroke width 240 dpi comprises 5 pixels or less (corresponding to 530 .mu. or less), the lines of the image become thick, and the degree of error increases as the stroke width decreases (in the drawing, 100% corresponds to the ideal value). As shown in FIG. 2, this is because, the electric field changes as shown by the dotted lines in FIG. 2 even if the potential is changed in the manner shown by the solid lines in FIG. 2, the strength of the electric field caused by the electrostatic latent image formed on a photosensitive drum is emphasized at the edges of the latent image.
FIG. 2A shows a change in the width of a line comprising 1 dot, and FIG. 2B shows a change in the width of a line comprising 5 dots. Deviation from the ideal value increases because of the influence of the edge effect as the stroke width decreases. These drawings show a case where a sleeve serving as the development electrode of a developer is close to a photosensitive drum and where the edge effect is initially reduced owing to the approach of the electrode. The drawings also show, however, that the influence of the edge effect on such a small stroke width as described above is not satisfactorily removed.